1. Field of the Invention
The invention relates to a method for inhibiting potato tuber sprouting.
2. Description of the Prior Art
Typically, tubers are harvested, allowed to suberize (i.e., allow the "skin" or periderm layer to toughen) at warm temperatures for about 10 days, then gradually cooled down to the storage temperature of about 10.degree. C. For the first 1-2 months after harvest, the tubers remain dormant and exhibit little inclination to sprout. However, after this period the tubers must be chemically treated to prevent sprouting from occurring, which causes numerous deleterious effects to the tubers. These effects include loss of fresh weight, the conversion of starch to sugars, and a decrease in the quality and appearance of tubers sold fresh. Sprouts and the surrounding tissue also contain elevated levels of toxic glycoalkaloids, which are not destroyed by cooking.
Chlorpropham (CIPC; 1-methylethyl-3-chlorophenylcarbamate) is currently used to control sprouting throughout the industry. Although CIPC has been used effectively for over three decades, questions concerning its toxicology have been raised, and it is currently under review by the Environmental Protection Agency. CIPC is known to be among the three pesticides found in the highest concentrations in the diet of the average American [Gartrell et al., J. Assoc. Off. Anal. Chem., 69:146-159 (1986)] and comprises over 90% of the total synthetic residues found in U.S. potatoes [Klocke et al., J. Chem. Ecol., 13:2131-2141 (1987)]. Therefore, a pressing need exists to find other, more environmentally acceptable sprout inhibitors for tubers.
Certain volatile monoterpenes have been shown to be potent growth inhibitors of plants and microorganisms and appear to be involved in allelopathic interactions between higher plants [Maugh, Science, 218:278 (1982)]. These compounds have low mammalian toxicities and are used in large quantities in flavorings, over-the-counter medications, and perfumes. Additionally, several of these compounds have been shown to be bactericidal or fungicidal and are active as insect repellants.
For many centuries, the Incas of South America and their descendants have buried potato tubers in pits that are layered with soil and the leaves of Muna plants that belong to the mint family Lamiaceae, and the genera Minthostachys and Satureja. This treatment prevents sprouting and excessive fresh weight loss and repels insect pests. These Muna plants contain copious amounts of essential oils that are substantially comprised of monoterpenes. Aliaga and Feldheim [Ernahrung, 9:254-256 (1985)] and Feldheim [Practicability and Mode of Action of Quality Storage of Potatoes After Harvest, In Report of a Lecture Given to the German Institute for Quality Research (Plant Nutrition Products), March, 1985, 6 pages] reported that the oil from the Muna plants was more effective than CIPC in inhibiting sprouting, fresh weight loss, and the incidence of rotted tuber parts over a period of 120 days. The authors also reported that the main components of the oil, including the monoterpenes .alpha.- and .beta.-pinene and limonene, and the oxygenated monoterpenes pulegone and menthone/isomenthone, are effective in this regard. However, no additional research was conducted concerning the effectiveness of other monoterpenes, particularly cineole, fenchone, or menthol, or other more easily obtained essential oils such as eucalyptus or peppermint.